1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and an image transferring unit for use in the same.
2. Description of the Background Art
A copier, printer or similar electrophotographic image forming apparatus includes a photoconductive drum or similar photoconductive element on which a toner image is to be formed. Arranged around the drum are a charger, a developing unit and a cleaning unit, which are image forming means joining in the formation of the toner image. An image transfer roller and an intermediate image transfer belt transfer the toner image from the drum to a sheet or recording medium.
The various members stated above each have a particular life determined beforehand in accordance with the material and the condition of use and are replaced when the life ends. Each member sometimes must be replaced before the end of life due to the deposition of impurities or scratches.
While the various members may be configured to be replaced individually, such a configuration not only increases the frequency of replacement and therefore time and labor necessary for replacement, but also sometimes scratch or otherwise damage the members around the member being replaced. In light of this, a current trend in the imaging art is toward an image forming apparatus in which the drum, charger, developing unit and cleaning unit are constructed into a single process cartridge bodily removable from the apparatus.
On the other hand, there has recently been proposed an image forming apparatus in which the drum, which is more expensive than the other members, is extended in life and replaceable independently of the other members. In this configuration, when the quality of an image transferred to a sheet is lowered due to scratches formed on the drum, the drum is replaced alone.
However, scratches formed on the drum are, in many cases, ascribable to an intermediate image transfer belt, sheet conveying belt, image transfer roller or similar image transferring member pressed against the drum. It is therefore likely that after the replacement of the scratched drum a new drum is also scratched in a short period of time and again lowers image quality unless the image transferring member, causative of the scratches, is replaced.
For example, in a color image forming apparatus including an intermediate image transfer belt., when impurities, including residual toner and paper dust, adhere to the outer surface of the belt, they scratch the above surface in the form of spots when pressed against the surface. As the impurities are repeatedly pressed against the drum, the spot-like scratches on the drum grow little by little and soon become stripe-like scratches. Such scratches capture a large amount of toner and appear in an image transferred to the sheet as black stripes.
On the other hand, when the impurities enter nips between the inner surface of the intermediate image transfer belt and rollers supporting it, they cause corresponding projections to appear on the outer surface of the belt. The projections of the belt also scratch the drum in the form of spots when pressed against the drum. These spot-like scratches also become stripe-like scratches.
In a color image forming apparatus of the type using a conveying belt for conveying a sheet in place of the intermediate image transfer belt, the conveying belt is not directly pressed against the drum during image formation because a sheet intervenes between the belt and the drum. However, the belt is pressed against the drum at the interval between consecutive sheets. As a result, when impurities adhered to the outer surface of the belt or enter nips between the belt and rollers supporting it and cause the outer surface of the belt to project, stripe-like scratches are also formed on the drum. This is also true with a monochromatic image forming apparatus including an image transfer roller pressed against the drum.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-227688 and 2002-108049.